Field of the Invention
The present invention relates to a lens apparatus, and particularly a lens apparatus including a cam mechanism for driving an optical member in an optical axis direction.
Description of the Related Art
A lens apparatus is provided with multiple lenses, such as a magnification lens and a focus lens, which are movable in an optical axis direction. The lens apparatus is generally provided with a mechanism for: rotating a cam ring using a motor or the like; and moving the lenses in the optical axis direction using a cam formed on the cam ring. The lens apparatus is further provided with a mechanism for rotating the cam ring, which is linked to an operation ring using a linkage member, when the operation ring is rotationally manipulated.
In a conventional practice, play small enough to have no influence on the optical performance is generally provided between the cam ring and a fixed barrel rotatably supporting the cam ring. Nowadays, as the number of pixels in an imaging sensor becomes larger, a permissible circle of confusion becomes smaller. In recent years, there have been a demand that backlash particularly in the optical axis direction be reduced to as close to zero as possible.
Japanese Patent Application Laid-Open No. 2010-145432 proposes a structure for preventing backlash from occurring around the optical axis due to the play, in which: at their one ends, a cam ring and a fixed barrel include tapered contact surfaces; at their opposite ends, a member for positioning the cam ring is provided; and a spring biases the cam ring in the optical axis direction. In addition, Japanese Patent Application Laid-Open No. H2-6915 proposes a structure for preventing backlash from occurring in the optical axial direction and around the optical axis, by providing an elastic member between a cam ring and a fixed barrel.
According to the conventional technique disclosed in Japanese Patent Application Laid-Open No. 2010-145432, the cam member is biased toward the tapered surface. Thus, the contact between the tapered surfaces makes a sliding area between them larger than the contact between vertical surfaces. Accordingly, the technique disclosed therein involves a risk of an increase in resistance due to the biasing, and a risk of an increase in operation torque. Meanwhile, Japanese Patent Application Laid-Open No. H2-6915 involves a risk of an increase in operation torque, because load is applied to sliding surfaces in the optical axis direction and around the optical axis.
As described above, since the cam ring of the lens apparatus is driven by the motor, an increase in operation torque may lead to an increase in size of the motor, which in turn invites an increase in costs and an increase in noise.